Electronic cards used within a data processing system require precise control of address, data and control signals (and in some cases clocks) for optimal performance. These signals are generated by electronic devices (e.g. semiconductor chips) accessing the contents of other semiconductor devices (e.g. memory chips) contained on the electronic cards. As the rise and fall times of these generated signals decreases to one (1) nanosecond (nsec) and below, the arrangement of circuit board traces and semiconductor devices on the electronic card strongly influences the shape of the signal presented to the semiconductor device. In most arrangements, the sharply changing voltages cause severe fluctuations on the signals as received by the semiconductor devices. The fluctuations can result in improper access to stored data, for example, causing errors in high speed computers, or resulting in lower performance.
The root cause of these signal fluctuations can usually be traced to the reflections of the signal pulses at the ends of the unterminated circuit board lines. The inductances and capacitances of the modules and connectors in the system can also contribute to the fluctuations, as well as mismatches between the impedances of the signal drivers and the signal traces. It is known that these fluctuations can be reduced by proper resistive termination of the circuit lines of the memory card, but this results in very large and undesirable power dissipation.
Other attempts at improving signal quality in high speed data processing systems are focussed on designs that minimize wire lengths. However, these types of designs are problematic in that the total path lengths from driver to (multiple) receivers are still longer than the signal rise times, and thus reflections from impedance discontinuities generally give rise to undesired signal distortions at the receiver(s).